function [ ] = mx_eyelink_get_next_data( )
%MX_EYELINK_GET_NEXT_DATA Fetches next data item from link buffer. Usually 
% called with <buf> = NULL, and returns the data item type. If the item is not
% wanted, simply ignore it. Otherwise, call eyelink_get_float_data() to read it
% into a buffer.
%
% Returns:
% 0 if no data, SAMPLE_TYPE if sample, else event type.
%
%----------------------------------------------------------------------------
% Copyright Eduardo Aponte 2013
% aponteeduardo@gmail.com